The present invention relates to a memory card and particularly to means for carrying out a function of store protection in a memory card having a memory which is partly or wholly rewritable.
In general, this type memory card has a plurality of memory integrated circuits (integrating circuits or memory elements having storage functions, which are generically called "memory IC" in this specification) mounted therein so that a large-capacity memory is logically provided by allocating a series of addresses to these memory ICs.
That is, this type memory card is widely used apparently as one memory, as an ROM card where games for a game machine are stored, or a detachable card where programs or parameters for a control unit are stored.
The structure of this type memory card has been standardized by JEIDA (for example, the IC Memory Card Guideline Vet. 4.1) in which specifications for interfaces to external devices (for example, the relation between the number of address lines and the arrangement of terminals, the relation between the kinds of control signals and the arrangement of terminals, and so on) are defined.
The fundamental structure of this type memory card is shown in FIG. 6A provided as a block diagram. In the drawing, the reference numeral 1 designates a memory, 2 a data line, 3 a lower address line, 4 a write control signal line, 5 a switch, 6 a read control line, and 7 a chip select signal line.
The memory 1, as described above, has a plurality of memory ICs and an address decoder for selecting one therefrom, but is regarded as a large-capacity memory (for example, 1 Mbyte memory) in the block diagram.
The data line 2 is generally provided as an 8-bit data line having external terminals D0 to D7 through which data are transmitted bidirectionally between an external device and the memory 1.
The address line 3 is provided as an address line having 20 lines (external terminals A0 to A19) corresponding to the 1 Mbyte storage capacity of the memory 1. Through the external terminals A0 to A19, address data are transmitted from the external device to the memory 1.
According to the JEIDA Standard, one address is constituted by 26 bits. Accordingly, external terminals A20 to A25 are not connected anywhere (as represented by NC in FIG. 6A) though they are defined as terminals.
The write control line 4 serves to transmit a write control signal for controlling write timing from the external device to the memory 1 through an external terminal WE. The switch 5 is connected between the external device and the memory 1 so as to be inserted in the middle thereof.
The switch 5 is constituted by a mechanical switch which can be changed manually from the outside of the memory card.
Accordingly, in the case where the switch 5 is changed to an open state, the write control signal cannot be transmitted to the memory 1 so that write protection is established in the memory card.
The read control line 6 is provided as a line through which a read control signal for controlling read timing is transmitted. The chip select signal line 7 is provided as a line through which a chip select signal for selecting one memory card while distinguishing the memory card from other memory cards is transmitted in the case where such memory cards are connected to a plurality of external devices of the same structure.
Accordingly, the timing of reading data from the memory card to an external device is controlled by the lines 6 and 7, whereas the timing of reading data from an external device to the memory card to the external device is controlled by the lines 4, 6 and 8.
In the aforementioned structure, an image of address space in this type memory card is shown in FIG. 6B.
A 64-M address space is constituted by 64 pieces of 1-M address spaces by repeating an address space of "0" to "1048575" (hereinafter merely called lowermost 1-M space, in which M is given to each M-byte in the drawing) 64 times. This is caused by the fact that the external terminals A20 to A25 for transmitting the upper bits of an address from an external device are not connected anywhere. Accordingly, the lowermost 1-M space and the other 1-M spaces can be shown by repeating one address space of the mounted memory 1, so that the memory 1 can be accessed even in the case where any one of the 1-M spaces is accessed.
As described above, the memory card is often used as an ROM card. In this case, it is a matter of course that the memory 1 may be constituted by an ROM, because the memory constituted by a mask ROM is advantageous from the double point of view of cost and capacity.
For the purpose of facilitating correction, updating and so on in the feature, however, a rewritable memory, such as an RAM, a PROM, an EEPROM, or the like, may be used as part of the ROM.
Accordingly, not only the memory card provided as an RAM card is rewritable but the memory card provided as an ROM card may be rewritable. Game makers or machine makers can maintain balance between cost saving caused by mass production and answering to diversified needs, by providing memory cards to end users after applying necessary correction/updating to the memory cards by writing additional data (and so on) into ROM cards.
Because this type memory card serves as an ROM card in the user's place, this type memory card must be prohibited from being carelessly rewritten by users. Therefore, a function of memory protection against writing is required. In the aforementioned structure in the prior art, the switch 5 fulfills this function.
In the prior art, however, the function of memory protection is selected by a mechanically operated switch. Accordingly, the switch may be operated by mistake consciously or unconsciously, so that the state of the memory card may be turned to a write-enabled state. When some write access to a memory card is made by careless operation of an apparatus (or the like) in the condition where the memory card is in a write-enabled state, and particularly when the lowermost address space frequently used is accessed, rewriting may occur easily to bring an inconvenient situation where data are destroyed because all address spaces are in most cases folded so as to correspond to respective areas of the mounted memory.
Furthermore, because ordinary users comprehend that such an ROM card is not rewritable, it is unnatural to users that the memory card has the read/write selection switch provided so as to be able to be operated for the maker's convenience.
Furthermore, in the case where data to be stored in the memory card are secret data, it is inconvenient that users other than a specific user can read such data easily.